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down in the thunderstorm.
N NOTE
For the particular thunderstorm cell shown in figure 5-4, the best
weather detection occurs with a 2° down tilt. The best tilt for
eliminating ground clutter is 1° up. When operating in the manual
mode, finding the right tilt angle often requires a compromise
between too much ground clutter and too little weather.
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MultiScan™ Radar Reflectivity
Figure 5-4 Thunderstorm Radar Displays versus Tilt Angle
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Thunderstorms MultiScan™ Radar
THUNDERSTORMS
INTRODUCTION
Thunderstorm development requires warm, humid, unstable air (warm
air near the surface and cooler air at higher altitudes). These conditions
are most common during the spring and summer months. In addition,
some type of lifting mechanism must be available to move the warm
moist air upward. This lifting motion might be provided by frontal
systems, air moving over mountains or even from gusts that originate in
other thunderstorms.
Figure 5-5 Severe Thunderstorm Activity
SINGLE-CELL THUNDERSTORM DEVELOPMENT
Single-cell thunderstorms are relatively rare but they do provide the
building blocks for more complex thunderstorm systems. Single-cell
thunderstorm development is described in figures 5-6 through 5-8.
Note that the entire life cycle from the time that the thunderstorm
begins to form until it dissipates is approximately one hour. Thus,
these thunderstorms are very dynamic, so rapid changes in associated
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MultiScan™ Radar Thunderstorms
weather conditions should be anticipated. When examining weather
at longer ranges, anticipate changes to the observed weather as the
aircraft approaches the thunderstorm location.
TOWERING CUMULUS STAGE
During the first stage of thunderstorm development, warm humid air
rises from the ground, creating an updraft. When this rising warm air
cools to the dew point, the air becomes saturated and is unable to hold
additional water vapor. Condensation occurs at this point and a cloud
begins to form (figure 5-6.)
Figure 5-6 Thunderstorm — Towering Cumulus Stage
MATURE STAGE
As the warm humid air continues to rise well above the freezing level,
the water becomes supercooled and ice crystals are formed. The
moisture that is held aloft by the strong updrafts forms an area of
potential energy know as thunderstorm vaulting (♦page 5-15). When
the water droplets and ice crystals grow large enough to overcome the
updraft, they begin to fall. This falling moisture will start dragging down
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Thunderstorms MultiScan™ Radar
the surrounding air, creating a downdraft. Eventually, this downdraft
is occurring simultaneously with the updraft. The existence of both
updrafts and downdrafts in a mature thunderstorm creates severe
turbulence (figure 5-7).
Figure 5-7 Thunderstorm — Mature Stage
DISSIPATING STAGE
During the dissipating stage of the storm, the downdraft eventually
becomes so large that it chokes off the supply of warm humid air that
is rising from the surface. When this occurs, the storm begins to die
and the rain decreases (figure 5-8).
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MultiScan™ Radar Thunderstorms
Figure 5-8 Thunderstorm — Dissipating Stage
MULTI-CELL THUNDERSTORMS
Most thunderstorms are actually multi-cell storms. That is, they are
composed of several cells in various stages of development. The
downdrafts from mature or decaying thunderstorm cells create a gust
front (a blast of cooler air along the ground) that can provide the
necessary lifting mechanism to cause new cells to develop (figure 5-9).
Multi-cell thunderstorms have the same kind of volatility as single-cell
storms and rapid change should be anticipated.
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Thunderstorms MultiScan™ Radar
CAUTION
Developing cells may not contain enough moisture to reflect radar
energy, but still contain severe turbulence. Whenever possible,
deviate to the upwind side of storms displayed on the radar.
Figure 5-9 Multi-Cell Thunderstorm
N NOTE
Studies have shown that thunderstorms tend to travel in the
direction of the winds around the 10,000 feet level. N NOTE
In a multi-cell thunderstorm environment, new cells will tend to form
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Collins Weather Radar operator’s guide(29)
